The present invention relates to an arrangement for understoichiometric gasification of spent liquor from chemical pulp production, comprising an upper reactor part, which upper reactor part is provided with a burner for the spent liquor and with an internally clad reactor jacket, and a lower separating part, comprising at least one wall, for separating a phase of solid and/or molten material, formed on gasification, from a phase of combustible gaseous material, which separating part is arranged to essentially convey the said phase of solid and/or molten material to a product liquid bath. The arrangement is principally intended for use in conjunction with the recovery of energy and chemicals from an expended cooking liquor from production of chemical paper pulp from a material containing lignocellulose.
For many years, the commercially dominant process for recovering energy and chemicals from so-called black liquor, which has been obtained in paper pulp production by the sulphate method, has generally been the so-called Tomlinson process, in which a so-called recovery boiler is used.
A more modern process is described in Swedish patent SE-C-448,173, which process is based on understoichiometric gasification/pyrolysis (i.e. with an oxygen deficit) of the black liquor in a reactor. The products are in this case a phase of solid and/or molten material, essentially comprising sodium carbonate, potassium hydroxide and sodium sulphide, and an energy-rich, combustible gas phase, essentially comprising carbon monoxide, carbon dioxide, methane, hydrogen gas and hydrogen sulphide. The mixture of solid/molten phase and gas phase is cooled and separated in a separating part connected to the reactor by means of direct contact with green liquor, the solid/molten phase dissolving in the green liquor. The green liquor is then conveyed for conventional causticizing for production of white liquor. The gas phase is used as fuel for generating steam and/or electrical power.
Other known reactors of the same type as in SEC-448,173 are disclosed, for example, in WO94/20677, WO93/0229 and WO93/24704. The separating part is usually arranged so that its outer walls constitute a continuation of an outer wall of the reactor, a constriction being present between the reactor and the separating part. The constriction, which usually has the shape of a truncated cone, bears a ceramic lining in the reactor. In connection with the constriction, there is also usually a downpipe with some form of cooling ring, which is arranged to spray water or green liquor into the flow of solid/molten material and gas from the reactor. The solid/molten material in the flow from the reactor is dissolved in the water or the green liquor.
It is known from WO95/35410 to use returned green liquor to create a thin wetting film on the inside of a downpipe at the outlet from the reactor.
It has been found that previously known constructions of the separating part, with its cooling ring, and of the transition between the reactor and the separating part entail a number of relatively serious problems. Thus, for example, thermal stresses arise in both the cooling ring and in the ceramic lining. The substantial constriction between reactor and separating part also leads to a turbulent flow in which smelt droplets recirculate to the lining and cooling ring. A related problem is that the cooling ring is very exposed to corrosion on account of the action of hot smelt, which can lead to cracks and leakage in the cooling ring, which in turn can cause very serious damage to the ceramic lining. Another problem is that of avoiding absorption of the gas phase, especially its carbon dioxide content, in green liquor formed. In such absorption, there is an undesired increase in the carbonate content and also hydrogen carbonate. For this reason, it is not advantageous to wet the inside of the downpipe with green liquor.
The object of the present invention is to reduce or eliminate the abovementioned problems by making available an arrangement for understoichiometric gasification of spent liquor from chemical pulp production which has an improved design of the transition between reactor and separating part. According to the invention, the cooling ring can be dispensed with, while the separating part is cooled efficiently and in a corrosion-inhibiting manner, and yet absorption of carbon dioxide in the green liquor formed is to the greatest possible extent avoided. According to one aspect of the invention, the transition is made essentially without constriction, creating conditions for a laminar outlet flow from the reactor.
The arrangement according to the invention is defined in Patent claim 1.
According to the invention, the arrangement for gasification of spent liquor comprises means for creating a cooling and protective film along at least one side, preferably the inside, of a wall for the separating part, the said means comprising a supply line for coolant liquid, which supply line, at its inlet end, is connected to a coolant liquid bath. The coolant liquid bath preferably consists of a condensate bath and is separate from a product liquid bath, i.e. separate usually from a green liquor bath.
The upper reactor part with ceramic lining is connected to a lower separating part cooled by a film of liquid, in which separating part smelt and combustion gas are separated. However, a considerable number of the reactions also take place in the separating part, which affords an extended reaction space. In the following description, however, this lower part is referred to only with respect to the separating stage.
According to one aspect of the invention, the coolant liquid film, the supply line and the coolant liquid bath are included in one circulation, the coolant liquid bath preferably consisting of a condensate bath through which the gas phase produced on gasification is bubbled.
According to another aspect of the invention, all or almost all the steel surfaces in the separating part are provided with liquid contact in the form of a liquid film or in the form of an adjoining liquid bath.
According to yet another aspect of the invention, the transition between reactor part and separating part is designed such that the constriction between them has an open area of at least 40% of the greatest internal area of the reactor part in the horizontal plane. The reactor part and its lining are connected to the upper end of the separating part, which is directly or indirectly cooled by the said coolant liquid film. In this way, the conventional bottom cone in the reactor can be largely dispensed with, and at the same time the conventional cooling ring is avoided.
According to yet another aspect of the invention, the lower part of the lining is formed with a self-supporting construction made of ceramic material stable against thermal shock.
According to a further aspect of the invention, the reactor operates at a pressure of 1.5-150 bar (abs.), preferably 1.5-50 bar, although atmospheric pressure is also conceivable. The temperature in the reactor can be 500-1600xc2x0 C., preferably 700-1300xc2x0 C.